Pen differentiation for touch display

ABSTRACT

An optical IR touch sensing apparatus can determine, based on output signals of light detectors, a light energy value for each light path across a touch surface, and generate a transmission value for each light path based on the light energy value. A processor can operate an image reconstruction algorithm on at least part of the thus-generated transmission values and determine a position of a touching object on the touch surface, an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface, and an occlusion compensation value for compensating the occlusion affect from other objects on the touch surface. Using these values, the processor can identify the type of object.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to techniques for detecting and identifying objects on a touch surface.

Description of the Related Art

To an increasing extent, touch-sensitive panels are being used for providing input data to computers, electronic measurement and test equipment, gaming devices, etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers. The GUI may be fixed or dynamic. A fixed GUI may e.g. be in the form of printed matter placed over, under or inside the panel. A dynamic GUI can be provided by a display screen integrated with, or placed underneath, the panel or by an image being projected onto the panel by a projector.

There are numerous known techniques for providing touch sensitivity to the panel, e.g. by using cameras to capture light scattered off the point(s) of touch on the panel, by using cameras to directly observe the objects interacting with the panel, by incorporating resistive wire grids, capacitive sensors, strain gauges, etc. into the panel.

In one category of touch-sensitive panels known as ‘above surface optical touch systems’ and known from e.g. U.S. Pat. No. 4,459,476, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid of intersecting light paths above the touch surface. Each light path extends between a respective emitter/receiver pair. An object that touches the touch surface will block or attenuate some of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the location of the intercept between the blocked light paths.

For most touch systems, a user may place a finger onto the surface of a touch panel in order to register a touch. Alternatively, a stylus may be used. A stylus is typically a pen shaped object with one end configured to be pressed against the surface of the touch panel. An example of a stylus according to the prior art is shown in FIG. 2. Use of a stylus 60 may provide improved selection accuracy and pointer precision over a simple finger touch. This can be due to the engineered stylus tip 160 providing a smaller and/or more regular contact surface with the touch panel than is possible with a human finger. Also, muscular control of an entire hand in a pen holding position can be more precise than a single finger for the purposes of pointer control due to lifelong training in the use of pens and pencils.

PCT/SE2016/051229 describes an optical IR touch sensing apparatus configured to determine a position of a touching object on the touch surface and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface. Using these values, the apparatus can differentiate between different types of objects, including multiple stylus tips, fingers, palms. The differentiation between the object types may be determined by a function that takes into account how the attenuation of a touching object varies across the touch surface, compensating for e.g. light field height, detection line density, detection line angular density etc. However, the determination of an object in this way becomes more difficult when other touching objects are close by, since they occlude a lot of detection lines (otherwise known as scanlines) passing through both the occluding objects and the object to be determined.

Therefore, what is needed is a way of improving the identification of objects touching an optical touch system that mitigates the above problem.

SUMMARY OF THE INVENTION

It is an objective of the disclosure to at least partly overcome one or more of the above-identified limitations of the prior art.

One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by means of a method for data processing, a computer readable medium, devices for data processing, and a touch-sensing apparatus according to the independent claims, embodiments thereof being defined by the dependent claims.

An embodiment provides a touch sensing apparatus, comprising: a touch surface, a plurality of emitters arranged around the periphery of the touch surface to emit beams of light such that one or more objects touching the touch surface cause an attenuation of the light; a plurality of light detectors arranged around the periphery of the touch surface to receive light from the plurality of emitters on a plurality of light paths, wherein each light detector is arranged to receive light from more than one emitter; and a processing element configured to: determine, based on output signals of the light detectors, a light energy value for each light path; generate a transmission value for each light path based on the light energy value; operate an image reconstruction algorithm on at least part of the thus-generated transmission values so as to determine, for each object; a position of the object on the touch surface, and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface, an occlusion compensation value indicative of the occlusion, by other objects on the touch surface, of light paths intersecting with the object, determine an object type of the object in dependence on the attenuation value and occlusion compensation value.

Another embodiment provides a method of determining a type of object in contact with a touch surface of a touch sensing apparatus, said touch sensing apparatus can include: a touch surface, a plurality of emitters arranged around the periphery of the touch surface to emit beams of light such that one or more objects touching the touch surface cause an attenuation of the light; a plurality of light detectors arranged around the periphery of the touch surface to receive light from the plurality of emitters on a plurality of light paths, wherein each light detector is arranged to receive light from more than one emitter; and said method comprising the steps of: determining, based on output signals of the light detectors, a light energy value for each light path; generating a transmission value for each light path based on the light energy value; operating an image reconstruction algorithm on at least part of the thus-generated transmission values so as to determine, for each object; a position of the object on the touch surface, and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface, an occlusion compensation value indicative of the occlusion, by other objects on the touch surface, of light paths intersecting with the object; determining an object type of the object in dependence on the attenuation value and occlusion compensation value.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings.

FIG. 1 is a top plan view of an optical touch apparatus.

FIG. 2 shows a cross-section of an IR optical touch apparatus according to the prior art.

FIG. 3 shows a cross-section of another IR optical touch apparatus.

FIG. 4 shows a light field of an IR optical touch apparatus.

FIG. 5a is a flow chart showing a touch determination process.

FIG. 5b is a flow chart showing a touch determination process using position compensation.

FIG. 5c is a flow chart showing a touch determination process using speed compensation.

FIG. 6 is a histogram showing measured attenuation of light beams from eight unique objects applied to the touch surface.

FIG. 7 is a histogram showing measured attenuation of light beams from two objects applied to the touch surface, a stylus and a finger.

FIG. 8 is a histogram showing measured attenuation of light beams from three objects applied to the touch surface, a first stylus, a second stylus, and a finger.

FIG. 9 is flow chart showing the process for determining an occlusion compensation value for a touching object.

FIG. 10a is diagram showing an example of occluding objects on a touch surface.

FIG. 10b shows the occluded angle ranges of a touching object.

FIG. 11 shows an object entering a light field.

FIG. 12 shows an attenuation value of an object during a ‘touch down’ and a ‘touch up’ event.

FIG. 13 is a graph showing measured attenuation of light beams by an object in proportion to the object's distance from a corner of the touch surface.

FIG. 14 is an attenuation map showing the relative attenuation of an object at each location on a corner portion of the touch surface.

FIG. 15 is a graph showing measured attenuation of light beams by an object in proportion to speed at which the object is moving across the touch surface.

DETAILED DESCRIPTION OF THE EMBODIMENT

The present disclosure relates to optical touch panels and the use of techniques for providing touch sensitivity to a display apparatus. Throughout the description the same reference numerals are used to identify corresponding elements.

In addition to having its ordinary meaning, the following terms can also mean:

A “touch object” or “touching object” may be a physical object that touches, or is brought in sufficient proximity to, a touch surface so as to be detected by one or more sensors in the touch system. The physical object may be animate or inanimate.

An “interaction” can occur when the touch object affects a parameter measured by the sensor.

A “touch” can denote a point of interaction as seen in the interaction pattern.

A “light field” can be the light flowing between an emitter and a corresponding detector. Although an emitter may generate a large amount of light in many directions, only the light measured by a detector from an emitter defines the light field for the emitter and detector.

FIG. 1 is a top plan view of an optical touch apparatus which may correspond to the IR optical touch apparatus of FIG. 2. Emitters 30 a are distributed around the periphery of touch surface 20, to project light across the touch surface 20 of touch panel 10. Detectors 30 b are distributed around the periphery of touch surface 20, to receive part of the propagating light. The light from each of emitters 30 a will thereby propagate to a number of different detectors 30 b on a plurality of light paths 50.

FIG. 2 shows a cross-section of an IR optical touch apparatus according to the prior art. In the example apparatus shown in FIG. 2, object 60 will attenuate light propagating along at least one light path 50. In the example shown of FIG. 2, object 60 may even fully occlude the light on at least one light path 50.

Light paths 50 may conceptually be represented as “detection lines” that extend across the touch surface 20 to the periphery of touch surface 20 between pairs of emitters 30 a and detectors 30 b, as shown in FIG. 1. Thus, the detection lines 50 correspond to a projection of the light paths 50 onto the touch surface 20. Thereby, the emitters 30 a and detectors 30 b collectively define a grid of detection lines 50 (“detection grid”) on the touch surface 20, as seen in a top plan view. The spacing of intersections in the detection grid defines the spatial resolution of the touch-sensitive apparatus 100, i.e. the smallest object that can be detected on the touch surface 20. The width of the detection line is a function of the width of the emitters and corresponding detectors. A wide detector detecting light from a wide emitter provides a wide detection line with a broader surface coverage, minimising the space in between detection lines which provide no touch coverage. A disadvantage of broad detection lines may be the reduced touch precision and lower signal to noise ratio.

As used herein, the emitters 30 a may be any type of device capable of emitting radiation in a desired wavelength range, for example a diode laser, a VCSEL (vertical-cavity surface-emitting laser), an LED (light-emitting diode), an incandescent lamp, a halogen lamp, etc. The emitters 30 a may also be formed by the end of an optical fibre. The emitters 30 a may generate light in any wavelength range. The following examples presume that the light is generated in the infrared (IR), i.e. at wavelengths above about 750 nm. Analogously, the detectors 30 b may be any device capable of converting light (in the same wavelength range) into an electrical signal, such as a photo-detector, a CCD device, a CMOS device, etc.

The detectors 30 b collectively provide an output signal, which is received and sampled by a signal processor 130. The output signal contains a number of sub-signals, also denoted “transmission values”, each representing the energy of light received by one of light detectors 30 b from one of light emitters 30 a. Depending on implementation, the signal processor 130 may need to process the output signal for separation of the individual transmission values. The transmission values represent the received energy, intensity or power of light received by the detectors 30 b on the individual detection lines 50. Whenever an object touches a detection line 50, the received energy on this detection line is decreased or “attenuated”. Where an object blocks the entire width of the detection line of an above-surface system, the detection line will be fully attenuated or occluded.

In an embodiment, the touch apparatus is arranged according to FIG. 2. A light emitted by emitters 30 a is transmitted through transmissive panel 10 in a manner that does not cause the light to TIR within transmissive panel 10. Instead, the light exits transmissive panel 10 through touch surface 20 and is reflected by reflector surface 80 of edge reflector 70 to travel along a path 50 in a plane parallel with touch surface 20. The light will then continue until deflected by reflector surface 80 of the edge reflector 70 at an opposing edge of the transmissive panel 10, wherein the light will be deflected back down through transmissive panel 10 and onto detectors 30 b. An object 60 (optionally having object tip 160) touching surface 20 will occlude light paths 50 that intersect with the location of the object on the surface resulting in an attenuated light signal received at detector 30 b. In an alternative embodiment shown in FIG. 3, emitters 30 a and detectors 30 b are arranged beyond the periphery of the panel and light is provided to the surface of panel 10 from beyond the edges of panel 10.

FIG. 4 shows the manner in which light travelling from emitters 30 a to detectors 30 b will form a light field 90 between reflector surfaces 80. In an embodiment, the top edge of reflector surface 80 is 2 mm above touch surface 20. This results in a light field 90 which is 2 mm deep. A 2 mm deep field is advantageous for this embodiment as it minimizes the distance that the object needs to travel into the light field to reach the touch surface and to maximally attenuate the light. The smaller the distance, the shorter time between the object entering the light field and contacting the surface. This is particularly advantageous for differentiating between large objects entering the light field slowly and small objects entering the light field quickly. A large object entering the light field will initially cause a similar attenuation as a smaller object fully extended into the light field. The shorter distance for the objects to travel, the fewer frames are required before a representative attenuation signal for each object can be observed. This effect is particularly apparent when the light field is between 0.5 mm and 2 mm deep.

Unless otherwise stated, the embodiments described in the specification apply to the arrangement shown in FIG. 2. However, some of these embodiments may also be applied to an arrangement shown in FIG. 3.

The signal processor 130 may be configured to process the transmission values so as to determine a property of the touching objects, such as a position (e.g. in a x,y coordinate system), a shape, or an area. This determination may involve a straight-forward triangulation based on the attenuated detection lines, e.g. as disclosed in U.S. Pat. No. 7,432,893 and WO2010/015408, or a more advanced processing to recreate a distribution of attenuation values (for simplicity, referred to as an “attenuation pattern”) across the touch surface 20, where each attenuation value represents a local degree of light attenuation. The attenuation pattern may be further processed by the signal processor 130 or by a separate device (not shown) for determination of a position, shape or area of touching objects. The attenuation pattern may be generated e.g. by any available algorithm for image reconstruction based on transmission values, including tomographic reconstruction methods such as Filtered Back Projection, FFT-based algorithms, ART (Algebraic Reconstruction Technique), SART (Simultaneous Algebraic Reconstruction Technique), etc. Alternatively, the attenuation pattern may be generated by adapting one or more basis functions and/or by statistical methods such as Bayesian inversion. Examples of such reconstruction functions designed for use in touch determination are found in WO2009/077962, WO2011/049511, WO2011/139213, WO2012/050510, and WO2013/062471, all of which are incorporated herein by reference.

For the purposes of brevity, the term ‘signal processor’ is used throughout to describe one or more processing components for performing the various stages of processing required between receiving the signal from the detectors through to outputting a determination of touch including touch co-ordinates, touch properties, etc. Although the processing stages of the present disclosure may be carried out on a single processing unit (with a corresponding memory unit), the disclosure is also intended to cover multiple processing units and even remotely located processing units. In an embodiment, the signal processor 140 can include one or more hardware processors 130 and a memory 120. The hardware processors can include, for example, one or more computer processing units. The hardware processor can also include microcontrollers and/or application specific circuitry such as ASICs and FPGAs. The flowcharts and functions discussed herein can be implemented as programming instructions stored, for example, in the memory 120 or a memory of the one or more hardware processors. The programming instructions can be implemented in machine code, C, C++, JAVA, or any other suitable programming languages. The signal processor 130 can execute the programming instructions and accordingly execute the flowcharts and functions discussed herein.

FIG. 5a shows a flow diagram according to an embodiment.

In step 510 of FIG. 5a , the signal processor 130 receives and samples output signals from detectors 30 b.

In step 520, the output signals are processed for determination of the transmission values (or ‘transmission signals’). As described above, the transmission values represent the received energy, intensity or power of light received by the detectors 30 b on the individual detection lines 50.

In step 530, the signal processor 130 is configured to process the transmission values to determine the presence of one or more touching objects on the touch surface. In an embodiment, the signal processor 130 is configured to process the transmission values to generate a two-dimensional estimation of the attenuation field across the touch surface, i.e. a spatial distribution of attenuation values, in which each touching object typically appears as a region of changed attenuation. From the attenuation field, two-dimensional touch data may be extracted and one or more touch locations may be identified. The transmission values may be processed according to a tomographic reconstruction algorithm to generate the two-dimensional estimation of the attenuation field.

In one embodiment, the signal processor 130 maybe configured to generate an attenuation field for the entire touch surface. In an alternative embodiment, the signal processor 130 maybe configured to generate an attenuation field for a sub-section of the touch surface, the sub-section being selected according to one or more criteria determined during processing of the transmission values.

In step 540, the signal processor 130 determines properties of the object at each touch location, including an attenuation value corresponding to the attenuation of the beams of light passing through the touch location resulting from the object touching the touch surface.

In one embodiment, the attenuation value is determined in the following manner: First, the attenuation pattern is processed for detection of peaks, e.g. using any known technique. In one embodiment, a global or local threshold is first applied to the attenuation pattern, to suppress noise. Any areas with attenuation values that fall above the threshold may be further processed to find local maxima. The identified maxima may be further processed for determination of a touch shape and a center position, e.g. by fitting a two-dimensional second-order polynomial or a Gaussian bell shape to the attenuation values, or by finding the ellipse of inertia of the attenuation values. There are also numerous other techniques as is well known in the art, such as clustering algorithms, edge detection algorithms, standard blob detection, water shedding techniques, flood fill techniques, etc. Step 540 results in a collection of peak data, which may include values of position, attenuation, size, and shape for each detected peak. The attenuation value may be calculated from a maximum attenuation value or a sum of attenuation values within the peak shape.

The attenuation value recorded for an object may vary due to noise, object angle, object material, or a number of other reasons. FIG. 6 is a histogram showing a count of attenuation values for each of eight unique objects applied to the touch surface. Each object demonstrates a roughly bell-shaped distribution of frequency of recorded attenuation values. It is clear from FIG. 6 that it is possible to differentiate between different objects using recorded attenuation values, particularly where multiple attenuation values for each object are recorded.

Certain objects may provide a wider distribution of attenuation values than others. FIG. 7 is a histogram showing measured attenuation of light beams from two objects applied to the touch surface, a stylus and a finger. Bell-shaped distribution of values 710 represents attenuation values for a specially designed stylus tip applied to the touch surface. Distribution 720 represents attenuation values for a population of different fingers applied to the touch surface. As people have different sized fingers and some fingers may be more oily that others, the range of possible attenuation values from objects in distribution 720 is much wider than the possible attenuation values for a specially designed stylus tip. Zone 730 represents attenuation values which are too small for the system to reliably record. In a typical example, zone 730 covers attenuation values smaller than 1.8*10⁻³ (Note: All attenuation values described in the present specification have units of mm⁻¹ but it is understood that attenuation may be measured in a number of different ways.) Depending on the touch resolution of the system, this may translate to objects smaller than 0.5 mm. Therefore, an embodiment of the disclosure includes a stylus tip configured to provide attenuation values in a range greater than values in zone 730 but smaller than the range of attenuation values occupied by distribution 720. E.g. 1.8*10⁻³<stylus tip attenuation<2.0*10⁻²

FIG. 8 is a histogram showing measured attenuation of light beams from three objects applied to the touch surface, a first stylus, a second stylus, and a finger. As in FIG. 7, bell-shaped distribution of values 810 represents attenuation values for a first specially designed stylus tip applied to the touch surface. Distribution 830 represents attenuation values for a finger applied to the touch surface. Zone 840 represents attenuation values which are too small for the system to reliably record. Bell-shaped distribution of values 820 represents attenuation values for a second specially designed stylus tip applied to the touch surface. Therefore, another embodiment of the disclosure includes a first stylus tip configured to provide attenuation values in a range greater than values in zone 840 but smaller than the range of attenuation values 820 occupied by a second stylus tip. The second stylus tip is configured to provide attenuation values in a range greater than values occupied by distribution 810 but smaller than the range of attenuation values occupied by distribution 830. E.g. 1.8*10⁻³<first stylus tip attenuation<7.2*10⁻³<second stylus tip attenuation<2.0*10⁻²

In step 542 of FIG. 5a , the signal processor 130 determines an occlusion compensation value of the object at each touch location. FIG. 9 shows a flow diagram for determining the occlusion compensation value for each touch location according to an embodiment.

In step 910, signal processor 130 determines all known objects within an occlusion region of the touch surface. A known object is typically a touch location identified in the present or a previous frame. In an embodiment, the occlusion region of the touch surface is determined by a radius around the touch location. The determination of known objects within a radius of the touch location may be determined by measuring the distance between the coordinates of the touch location and the coordinates of the respective known object. If the distance is less than the value used to determine the radius, the known object is determined to be within the occlusion region.

In some embodiments, the occlusion region is the entire touch surface. In other embodiments, the occlusion region may be limited to a predefined cell of a plurality of cells covering the touch surface, wherein a cell is a portion of the touch surface. In this embodiment, the occlusion region may be limited to the cell occupied by the touch location.

Steps 920-940 may be executed for each known object in the occlusion region.

FIG. 10a shows an example set of objects 250, 260, 270, 280, and 290 in contact with an example touch surface.

FIG. 10b shows the occlusion of light at touch location 250 on the touch surface with respect to angle φ resulting from occluding objects 260, 270, 280, and 290. Occlusion shadow 265′ shows the shadow resulting from objects 260 and 270. Although the figure shows the accumulated occlusion where the objects overlap, occlusion level 262 shows the maximum occlusion used to determine occlusion of an object. This is a natural result of the fact that light cannot typically be occluded more than once by opaque objects. Occlusion shadow 280′ shows the shadow resulting from larger object 280. Occlusion shadow 290′ shows the shadow resulting from object 290. Occlusion shadows 265″, 280″, and 290″ are the corresponding occlusion shadows resulting from detection lines that must be treated as occluded as they also pass through the position of the respective occluding object, but from the far side of touch location 250.

In step 920, the angle φ1 between axis N and the vector between the first respective known object 260 and the touch location 250 is determined. In an embodiment, the centroid of the area of interaction between the object and the touch surface is used to determine co-ordinates of touching objects. In an embodiment, the angle may be calculated using the co-ordinates of the known object 260 and touch location 250 and ordinary Euclidean geometry.

In step 930, the size of the first respective known object 260 is determined. In one embodiment, an estimated size of the first known object 260 may already be known by the touch determination system or may be determined as a function of the reconstruction image generated by the signal processor 130, e.g. in dependence on the portion of the reconstruction image occupied by the first known object 260. For the purposes of the present embodiment, the size of an object is the diameter of the object in the plane of the touch surface as viewed from the touch location 250. i.e. The width of the silhouette of the known object as viewed from the touch location 250. Where first known object 260 is substantially circular, this diameter may be largely consistent regardless of the angle between the touch location 250 and first known object 260. Where first known object 260 is irregular, the diameter of the object in the plane of the touch surface as viewed from the touch location 250 may vary considerably in dependence on the orientation of first known object 260 and/or the angle between the touch location 250 and first known object 260. For smaller objects, this variation may be ignored, but for larger objects (e.g. a rectangular board eraser), the variation in the diameter of the large object as viewed from the touch location 250 may be as much as three times larger from one angle than from another. In one embodiment, the size of an object is determined as a function of the width, length, and azimuth of the object, wherein the length of the object is defined as the major axis of the object, the length of the object is defined as the minor axis of the object, and wherein the azimuth is the angle between the major axis of the object and axis N.

In an embodiment, the length, width, and azimuth are used to determine a rectangular bounding box B around the shape of the known object. The length, width of the known object defines the length and width of bounding box B, and the azimuth defines the rotation of bounding box B. The co-ordinate of the four corners of the bounding box are then determined and the two co-ordinates defining the minimum (B_(min)) and maximum angle (B_(max)) to the touch location 250 with respect to axis N are used to determine the diameter of the object in the plane of the touch surface as viewed from the touch location 250, and therefore the size of the object.

In step 940, the angular range over which the light reaching touch location 250 is occluded by first known object 260 is calculated. In an embodiment, the angular range φΔ is determined to be: φΔ=2*tan⁻¹ (size/(2*distance between objects))

In an embodiment, where the ratio between the known object size and the distance between the known object and the touch location is larger than a threshold, angular range φΔ is determined to be: φΔ=size/distance between objects

In one embodiment, the threshold is between 0.15 and 0.5. In an embodiment, the above threshold is 0.3.

In step 950, the angular range for all known objects is aggregated to determine the full occlusion for all the known objects. In an embodiment, the angular range for each of the known objects is summed to generate a summed angular range. For overlapping angular ranges (e.g. occlusion shadow 265 of FIG. 10b ), the overlapping portion is used only once for the generation of the summed angular range. i.e. The determined occlusion has a maximum of occlusion maximum 262. This is to ensure that the occlusion compensation value is not distorted by angular ranges where light has already been occluded once.

In step 960, the summed angular range is normalised to generate an occlusion ratio. In one example, the summed angular range is normalised by dividing the summed angular range by pi radians or 180 degrees, depending on the angular units used to determine the summed angular range. The need for this normalisation step is shown in FIG. 10b , where occlusion shadows 265″, 280″, and 290″ must also compensated for. In an alternative embodiment, the step of normalization is performed as part of step 940 for each angular range φΔ.

In step 970, an occlusion compensation value for the touch location is generated in dependence on the occlusion ratio. Because the attenuation of an object at touch location 250 will decrease in dependence on the occlusion ratio, it is useful to use this occlusion ratio to generate the occlusion compensation value. As the detection lines occluded by other objects cannot be used to determine the attenuation caused by the object at touch location 250, the measured attenuation is reduced in proportion to the number of occluded detection lines. Consequently, the measured attenuation can be compensated using the occlusion ratio. In an embodiment, the compensated attenuation value is equal to the occlusion ratio.

In an alternative embodiment, the occlusion compensation value is generated in dependence on the number of known objects present on the touch surface alone.

In another alternative embodiment, the occlusion compensation value is generated just in dependence on the number of known objects present on the touch surface within a selected distance of the object.

In step 548 of FIG. 5a , the signal processor 130 determines a compensated attenuation value of the object at each touch location in dependence on the attenuation value and the occlusion compensation value. In an embodiment, the compensated attenuation value is determined by dividing the attenuation value determined in step 540 by the occlusion compensation value determined in step 542/970.

In an embodiment, signal processor 130 is configured to store a plurality of object IDs in memory, each object ID having an associated attenuation value range. In the following example, three object types with associated Object IDs are shown.

Object ID: 001 002 003 Object type: Stylus Thick Stylus Thin Finger Output type: Think Blue Ink Thick Red Ink Thick Black Ink Attenuation Max: 2.0 * 10⁻² 7.2 * 10⁻³ Attenuation Min: 7.2 * 10⁻³ 1.8 * 10⁻³ 2.0 * 10⁻²

In an embodiment, each Object ID has an attenuation value range, defined by an Attenuation Max value and an Attenuation Min value. The Object IDs may optionally include further values defining properties of an associated object, including a recognised object type, an output type (e.g. a brush type, ink colour, selection type, etc.)

In step 550, signal processor 130 matches each touch location to an Object ID. This is done by matching the compensated attenuation value of each touch location to the range of the matching Object ID. i.e. A touch location with an attenuation value of 1.2*10⁻² will be matched to Object ID 001. In one embodiment, an Object ID exists with a range for all values above a specific value. This allows all objects with a compensated attenuation value above the usual ranges of the Object IDs to be identified using the same ‘default large object’ Object ID. Similarly, in one embodiment, an Object ID exists with a range for all values below a specific value allowing very low compensated attenuation value objects to be identified with a generic ‘default small object’ Object ID.

In step 560, signal processor 130 outputs the touch data, including the touch locations and corresponding Object IDs for each location.

In an alternative embodiment to that shown in FIG. 5a , the attenuation compensation value determined in step 540 is applied to the Attenuation Max value and an Attenuation Min value of the respective Object IDs, instead of directly to the attenuation value of the touch location. In an example of this embodiment, each of the Attenuation Max value and an Attenuation Min value of an Object ID is multiplied by the Attenuation Value to determine a Compensated Attenuation Max value and Compensated Attenuation Min value for the Object ID. The attenuation value is then compared against the Compensated Attenuation Max value and Compensated Attenuation Min value to determine the matching Object ID.

When matching an attenuation value of a touch to an object ID, it is important to use a stable attenuation value which correctly reflects the attenuation of the light caused by the object once it is in contact with the surface. In an ‘above surface’ system such as the embodiment shown in FIG. 2, light field 90 has a depth and so the object must travel a distance through the light field before contacting the touch surface. Consequently, a period of time between when the object enters the light field and when the object contacts the touch surface exists when the attenuation caused by the object is likely to be increasing. Any attenuation values measured during this period will likely not accurately reflect the light attenuation of the object once it is contacting the touch surface. In one embodiment of the disclosure, step 540 is delayed until an attenuation value of an object is determined to be stable. In one embodiment, the attenuation value of an object is determined to be stable once it has not changed greater than 10% each frame for at least three frames.

As an object is lowered into the light field, it occludes increasingly more light. As a consequence, the attenuation of light caused by the object increases until the object has hit the touch surface. The gradient of attenuation (i.e. the rate of change of the attenuation) is therefore positive as the object travels towards the touch surface until it flattens out when the object is in contact with the surface. FIG. 13 shows an object 60 with tip 160 having traveled into light field 90 for a distance of h_(max)−h. FIG. 13 shows an attenuation value 1020 of an object during a ‘touch down’ event 1040 (i.e the application of a touching object to the touch surface) and a ‘touch up’ event 1050 (i.e lifting the touching object off and away from the touch surface). A corresponding height h (shown as line 1010) of the object from the touch surface is also shown. The line 1030 showing the attenuation gradient (i.e. the rate of change of change of the attenuation value with respect to time) shows a typical attenuation gradient signature for both touch down and touch up events. An attenuation gradient signature is the shape of the attenuation gradient values during a touch down or touch up event.

Therefore, in an embodiment of the disclosure, signal processor 130 is configured to determine that an object attenuation value is stable and/or that a touch down event has occurred in dependence on an attenuation gradient signature (shown at time 1040 in FIG. 13) of an event. In an embodiment, the attenuation gradient signature corresponding to a touch down event is a first period of a first attenuation gradient, a second period of higher attenuation gradient, and a third period of attenuation gradient lower than the second period.

In one embodiment, a touch down event determined to have occurred once object attenuation value has exceeded a first attenuation value threshold. However, a determination that a touch down event has occurred is possible before this threshold is met, using the above method. Where the object attenuation value is below the first attenuation value threshold but an attenuation gradient signature is observed having a higher attenuation gradient equal to or greater than 20% of the first attenuation value threshold over a single frame, the object attenuation value may be determined to be stable and/or that a touch down event has occurred.

During a ‘touch up’ event, an attenuation value of the object decreases as the object is lifted out of the light field. Similarly to the above, the attenuation gradient signature of this event (shown at time 1050 in FIG. 13) can be recognized and actioned accordingly. Therefore, in an embodiment of the disclosure, signal processor 130 is configured to determine that an object attenuation value is reduced to zero and/or that a touch up event has occurred in dependence on an attenuation gradient signature of an event. In an embodiment, the attenuation gradient signature corresponding to a touch up event is a first period of a first attenuation gradient, a second period of negative attenuation gradient, and a third period of attenuation corresponding to the first attenuation gradient.

In one embodiment, a touch up event is determined to have occurred once the object attenuation value is determined to have dropped below a second attenuation value threshold. However, a determination that a touch up event has occurred is possible before this threshold is met, using the above method. Where the object attenuation value is above the second attenuation value threshold but an attenuation gradient signature is observed having a negative attenuation gradient equal to or greater than 20% of the second attenuation value threshold over a single frame, a touch up event may be determined to have occurred.

In an embodiment, the attenuation gradient values required to trigger touch up/down events for an object may be scaled in dependence on the presence of other occluding objects in close proximity to the object. In an example, the attenuation gradient of the second period of a signature is scaled up to require an even larger value to trigger a touch down event for an object in close proximity to other occluding objects on the touch surface. In one embodiment, the higher attenuation gradient is scaled linearly occurred to the number of additional touches within a radius of up to 10 cm. The radius may be chosen in dependence on the screen size, touch resolution, and environmental noise.

‘Hooks’ are a problem observed in the flow of co-ordinates of user touch input over time when the user is providing rapidly changing touch input. E.g. When drawing or writing. An example of a ‘hook’ is where the user finishes drawing a stroke, lifts the touch object from the surface of the panel and rapidly changes the direction of movement of the touching object to begin drawing the next stroke. The ‘hook’ is a small artifact seen at the end of the stroke pointing in the new direction of the user's touch object. A method of minimizing hooks is proposed. In an embodiment of the disclosure, once a negative attenuation gradient has been observed, the touch coordinates will not be updated with the object's position and the coordinates of the object's position are stored. If the object attenuation value drops below a threshold value, the stored coordinates are discarded and a ‘touch up’ event is signaled. If the object attenuation value does not drop below a threshold value and a positive attenuation gradient is subsequently observed, the stored touch coordinates for the intervening period will be output and the touch coordinates will continue to be output as before. In an embodiment, the method is only used when the direction of movement of the object contacting the touch surface in the plane of the touch surface is changing. In this embodiment, a vector α from a last touch coordinate of the object to a current coordinate is determined. A second vector β from a touch coordinate previous to the last coordinate to the last coordinate is determined. Vectors α and β allow a determination of the direction the interaction is moving and how it is changing. A rapid change of direction of the object may result in α scalarproduct β<0. In one embodiment, if this condition is met, it may be determined that the direction of movement of the object contacting the touch surface has significantly changed and the above method for minimizing hooks is then applied.

Although the attenuation value of an object provides information regarding the light attenuated by the object touching the surface, some embodiments of the disclosure require that the attenuation value be compensated in order to provide a true reflection of the nature and/or position of the object.

In one embodiment of the disclosure shown in FIG. 5b , the compensated attenuation value of an object is determined in dependence on the attenuation value of the object determined in step 540, an occlusion compensation value determined in step 542, and a position compensation value determined in step 544.

In certain arrangements of the system shown FIG. 1, certain positions on the touch surface are likely to result in lower attenuation values than others for equivalent objects. In particular, attenuation values towards the edge of the screen are likely to be lower than in the centre. A variety of factors may cause this to be the case. One is that efficient implementations of certain tomographic reconstruction algorithms make approximations resulting in lower reconstructed attenuation values towards the edges of the panel. In one example, attenuation values in a corner of a panel may be as low as 30% of attenuation values located at the centre of the panel for an equivalent object. FIG. 13 shows a graph of attenuation values (shown as relative attenuation) of an object touching a rectangular touch surface relative to the distance of the object from a corner of the touch surface. Consequently, an embodiment of the disclosure provides that the position compensation value is a function of at least the position of the object on the touch surface. In one embodiment, the position compensation value is determined as a function of the distance from a central point on the touch surface to the touch position. Alternatively, the compensation value may be determined as a function of the distance from the nearest corner of the touch surface to the touch position.

The relationship between the position of the touch and a required position compensation value may be a complex function of the geometry of the emitters and detectors. FIG. 14 shows a heat map of a corner of a rectangular touch surface showing relative attenuation of a touching object. When touching at co-ordinate (0,0), the object generates relatively little attenuation. When touching at co-ordinate (10,15), a much larger amount of attenuation occurs.

Consequently, an embodiment of the disclosure provides calculating a position compensation value as a function of the position of the corresponding touch on the touch surface. An alternative embodiment describes using a compensation map to determine a position compensation value given a position on the touch surface. The compensation map may include a 2D image corresponding to the dimensions of the touch surface with pixel values corresponding to position compensation values. A touch position is then used to determine the corresponding pixel on the compensation map and the pixel value at that position provides the corresponding position compensation value. In an embodiment, the compensation map has a resolution lower than or equal to the touch resolution of the touch determination system. The compensation map is preferably generated in advance but may also be generated dynamically as a function of environmental and performance variables. In one embodiment, the compensation map is generated manually or using a calibration robot during the manufacturing phase.

Another variable which may affect the recorded attenuation of a touch object is the speed at which the touching object is moving across the touch surface. The light attenuation of each light path is recorded sequentially over a series of frames. Therefore, a sufficiently fast moving object may have moved away from a specific position before the attenuation of all light paths intersecting the position have been measured. Consequently, a moving object may generate a weaker attenuation signal. FIG. 15 shows a graph of recorded attenuation values of an object relative to the speed of the object across the touch surface. A relationship can be seen showing that a faster moving object is likely to generate a weaker attenuation value. Therefore, in one embodiment of the disclosure shown in FIG. 5c , the compensated attenuation value of an object is determined in dependence on at least the attenuation value of the object determined in step 540, an occlusion compensation value determined in step 542, and a speed compensation value determined in step 546. As the relationship between the speed of an object and the recorded attenuation value may also be complicated by the position of the moving object on the touch surface, an embodiment of the disclosure provides determining a compensation value as a function of both the position and speed of the object on the touch surface. 

What is claimed is:
 1. A touch sensing apparatus, comprising: a touch surface, a plurality of emitters arranged around a periphery of the touch surface, the plurality of emitters configured to emit beams of light such that one or more objects touching the touch surface cause an attenuation of the light; a plurality of detectors arranged around the periphery of the touch surface, the plurality of detectors configured to receive light from the plurality of emitters on a plurality of light paths, wherein each detector in the plurality of detectors is arranged to receive light from more than one emitter in the plurality of emitters; and a hardware processor configured to: determine, based on output signals generated by the plurality of detectors, a light energy value for each light path of the plurality of light paths; generate a transmission value for each light path of the plurality of light paths based on the light energy value; from at least part of the thus-generated transmission values, determine for each object; a position of the object on the touch surface, and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface, an occlusion compensation value indicative of the occlusion, by other objects on the touch surface, of light paths intersecting with the object, determine an object type of the object in dependence on the attenuation value and occlusion compensation value.
 2. The touch sensing apparatus of claim 1, wherein other objects on the touch surface are occluding objects and the occlusion compensation value is determined in dependence on the number of occluding objects.
 3. The touch sensing apparatus of claim 2, wherein the occlusion compensation value is determined in dependence on the number of occluding objects within a selected distance of the object.
 4. The touch sensing apparatus of claim 1, wherein the occlusion compensation value is determined as a function of the size and angle with respect to the object, of the occluding objects.
 5. The touch sensing apparatus of claim 4, wherein the size an occluding object is determined as a function of a width, length, and azimuth of the occluding object.
 6. The touch sensing apparatus of claim 1, wherein the image reconstruction algorithm is an algorithm for transmission tomography.
 7. The touch sensing apparatus of claim 1, the processing element further configured to store a plurality of object IDs, each having an associated attenuation value range.
 8. The touch sensing apparatus of claim 7, the processing element further configured to: determine an occlusion compensated attenuation value for the object in dependence on the attenuation value and the occlusion compensation value; identify an object ID with an attenuation value range corresponding to the occlusion compensated attenuation value of the object and associating the object ID with the object.
 9. The touch sensing apparatus of claim 8, wherein the identifying step is not performed by the processing element until the attenuation value is determined to be stable.
 10. The touch sensing apparatus of claim 9, wherein the attenuation value is determined to be stable when the rate of change of the attenuation value is below a predefined threshold.
 11. The touch sensing apparatus of claim 9, wherein the attenuation value is determined to be stable when the rate of change of the attenuation value is determined to rise above a first threshold and then subsequently drop below a second threshold, wherein the second threshold is lower than the first threshold.
 12. The touch sensing apparatus of claim 1, the attenuation value is generated in dependence on the attenuation of the light resulting from the object touching the touch surface and a compensation value.
 13. The touch sensing apparatus of claim 12, wherein the compensation value is a function of at least the position of the object on the touch surface.
 14. The touch sensing apparatus of claim 12, the compensation value being the value at a position on a compensation map corresponding to the position of the object on the touch surface.
 15. The touch sensing apparatus of claim 12, wherein the compensation value is a function of at least the speed of the object across the touch surface.
 16. The touch sensing apparatus of claim 15, wherein the compensation value is proportional to the speed of the object across the touch surface.
 17. A method of determining a type of object in contact with a touch surface of a touch sensing apparatus, said touch sensing apparatus comprising: a touch surface, a plurality of emitters, arranged around the periphery of the touch surface, configured to emit beams of light such that one or more objects touching the touch surface cause an attenuation of the light; a plurality of detectors, arranged around the periphery of the touch surface, configured to receive light from the plurality of emitters on a plurality of light paths, wherein each detector in the plurality of detectors is arranged to receive light from more than one emitter in the plurality of emitters; and said method comprising the steps of: determining, based on output signals generated by the plurality of detectors, a light energy value for each light path of the plurality of light paths; generating a transmission value for each light path of the plurality of light paths based on the light energy value; from at least part of the thus-generated transmission values, determine for each object; a position of the object on the touch surface, and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface, an occlusion compensation value indicative of the occlusion, by other objects on the touch surface, of light paths intersecting with the object; determining an object type of the object in dependence on the attenuation value and occlusion compensation value. 